In flameless absorption spectrophotometer measurements, a sample of material to be analyzed is placed in a tubular graphite furnace which is electrically heated so that the sample first dries, then ashes, and is eventually heated to the point at which the various elements in the sample become atomized. When the particular element or elements to be measured have been released into their atomic state, a light beam originating from a resonant line-emitting source is passed through the heated furnace tube and into a monochromator and detector, the circuitry of which determines the concentration of the desired element in the sample by measuring the intensities of appropriate portions of the resulting beam after the desired atomic element has absorbed its characteristic resonant lines.
The temperature to which the graphite tube is to be heated is important since insufficient heating may atomize portions of the sample but not the desired element to be measured, and excessive heat may atomize such more than is necessary and release undesired atoms that may interfere with the desired measurements. Generally, such graphite tube measurements are made by an optical pyrometric detector that is positioned to observe a spot on the exterior surface of the heated graphite tube and near its center where the sample is injected and at the location of the highest heat of the tube. The pyrometer output is then scaled to a particular factor to approximate the interior heat of the tube, and this scaled signal is then applied to a heat controller which controls the electrical current through the graphite tube to reach and maintain the desired atomizing temperature of the element to be measured.
While pyrometric measurements have generally proved to be quite satisfactory, there are difficulties that result in erroneous temperature measurements when a spot on the tube exterior is measured. For example, the graphite tube is normally supported between electrodes which, in turn, are held between large cooling jackets provided to prevent heat damage to associated hardware. The spacing between the surrounding electrodes must be sufficiently large to prevent electrical short-circuiting between the electrodes but must be maintained small to prevent heat escape from the graphite tube. It is, therefore, extremely difficult to provide sufficient space to image a pyrometer against the graphite tube surface. Furthermore, to eliminate oxygen from the area of the heated tube and thereby prevent burning of the tube, it is necessary to circulate an inert gas around the outer surface of the tube. Because of the necessary electrical insulating spacing between the electrodes and because there is an additional opening in the tube and one electrode to permit insertion of a sample into the graphite tube, a considerable amount of this inert gas will normally escape into the atmosphere and it is therefore necesssary that large quantities of this inert gas be admitted into the area between the exterior of the graphite tube and the interior of the electrodes. This flow of inert gas causes variations in temperature of the external surface of the graphite tube and consequently an inaccurate temperature measurement by a pyrometer detector viewing that external tube surface.